1. Field of the Invention
The present invention relates, in general, to data communications, and, more particularly, to communicating data between a transponder and an inductively coupled base station.
2. Relevant Background
Smart cards are wallet-sized devices that allow their holder to access and manipulate selected data based on the information held in the smart card. In contrast with conventional transaction cards with data stored on a magnetic stripe applied to the transaction card, a smart card includes active data processing components to support multiple applications, more functions, more features, and higher transaction speeds. Typical applications include automatic fare collection, ticketless travel, health care, access control, manufacturing automation, point-of-sale transactions, and on-line payments or fund transfers. Smart cards also enable a single card to support multiple applications. The smart card industry is expected to grow rapidly as the need for electronic commerce increases.
Two basic classes of smart cards exists: those that operate through physical contact with a terminal and those that operate through radio frequency (RF) data transmission between the smart card and the terminal. The latter class are referred to as xe2x80x9ccontactlessxe2x80x9d smart cards. Contactless implementations are preferred because they ease use, simplify hardware, and in theory last longer with less maintenance because of the lack of wear and tear caused by physical contact.
One contactless technology uses a host computer that writes data to and reads data from the card through a controller. The card is alternatively referred to as a transponder. The controller communicates with the host computer via a serial or parallel connection. The controller translates signals and data received from the host computer into signals that are to be communicated to the transponder.
When commanded to communicate with a transponder the controller continuously produces an unmodulated carrier from its antenna coil. The transponder contains a coil of wire that derives energy from the controller signal to power the electronics on board the transponder. Power supply circuitry coupled to the transponder""s coil rectifies and filters the received energy to provide stable voltage potentials for the transponder. However, in general the transponder power supply is quite fragile in that the total quantity of energy transferred from the controller is quite limited as compared to many electronic applications.
Signal and data information can be communicated by modulating the carrier using, for example, frequency shift keying (FSK). While the card remains in a sufficiently intense controller field it transmits signal and data information back to the controller using, for example, phase shift keying (PSK) In this manner the controller and transponder remain in continuous communication while the transponder remains in the controller field. It is important that the communication link remain continuous to avoid unnecessary latency in the communication and to avoid corrupt data transfers due to interrupted communication resulting from transponder shutting down or resetting due to lack of power.
In one contactless technology the controller and transponder are magnetically coupled. This technology is sometimes referred to as inductive signaling. The transponder does not emit an RF signal, but instead modulates the impedance of its antenna coil. The changing impedance can be detected at the controller""s antenna coil as a change in mutual inductance. In other words, as the transponder modulates the impedance of its coil, the impedance of the controller""s own coil changes in a detectable manner.
Prior transponder circuits modulate the coil impedance using an impedance that is switched in and out across the coil. This impedance consumes current when it is switched in. Hence, while the transponder is sending data, the current drain caused by the impedance modulation circuit can be substantial. As a result, the transponder power supply can be pulled down or loaded by the impedance modulation circuit to a point where one or more electronic components no longer have sufficient voltage to operate. A need exists for a method and system for communicating information from a transponder that does not interfere with the transponder""s power supply.
Briefly stated, the present invention involves a method and system for communicating data between controller and a transponder where the processor is powered by current received from an antenna. The controller is magnetically coupled to the transponder to sense impedance variations in the transponder. The impedance of the transponder is altered by varying current used by a processor within the transponder. The processor current is controlled by varying the processor clock rate.